Navagational lights used in airports in the United States must conform to certain minimum standards set by the Federal Aviation Administration. Comparable standards exist in other countries proscribed by the appropriate agency having jurisdiction over airports in that particular country. In the United States FAA Advisory Circular AC 150/5435-46 sets forth the specifications for elevated runway edge lights identified as FAA L-862 and elevated runway threshold lights identified as FAA L-862E.
In each instance the light, while providing lighting through a full 360.degree. area, contains two oppositely disposed areas of high intensity light, 180.degree. apart, thus enabling pilots to identify the threshold and edges of the runway more readily.
These areas of high intensity are created by the use of an inner and an outer optical prism. The outer prism is usually a single hemispherical-shaped, vertically elongated, transparent glass dome, thus providing weatherproofing for the lamp inside. Inside this prism is located a second prism, usually in two parts, which surrounds the light bulb.
The inner prism may be transparent or may be colored. For example, in some cases one half of the inner prism will be green and the other half will be red, or some other color combination.
The inner prism tends to compress the light rays passing through it in a vertical direction, whereas the portion of the outer prism through which the same light rays pass are compressed in a horizontal direction, thus resulting in a single beam of bright light emanating from either side of the light.
It is essential that these prisms be precisely aligned so that the optimum optical results are achieved. During normal operation it is necessary to change the high-intensity quartz light bulb from time to time in the field and this requires the removal of the outer prism, separation and removal of the two inner prisms, and any metal bands or other units that are used to hold the assemblage in place, in order to reach the light bulb.
In prior art devices, as many as six separate operations must be performed before the light source can be replaced.
Not only is this inconvenient for the maintenance worker, especially in adverse weather conditions such as snow and rain, but can result in the loss of components, such as in the snow, and in the case of colored inner prisms, sometimes can result in reverse assemblage of the units, which is not observed until the lights are again turned on at night time.